1. Field of the Invention
This invention relates to a method of non-destructive examination by electromagnetic induction of metallic material in cylindrical solid or tubular form. It relates also to apparatus for this purpose.
2. Description of Related Art
By electromagnetic induction, eddy currents are generated in the metallic material under examination as a result of AC excitation of a primary coil or coils, and the perturbation of the eddy currents caused by variations in the properties of the material gives rise to modifications of the associated magnetic field which are detectable by a secondary coil or coils, or by other suitable detector means.
For the examination of metallic material in cylindrical form, most commonly tubular cylindrical material such as pipe or tube, a close electromagnetic coupling of the primary coil with the material, as distinct from a remote field arrangement, is effected by sizing the coil to fit closely, even contiguously, around the outer diameter of the material, or around the inner diameter if the form is tubular. Such coupling is effective over the full circumference of the material and is an aid against the phenomenon of so-called "lift-off" which is a variability of the coupling giving rise to interfering signals in the conduct of the examination. However, with close electromagnetic coupling there is precluded the arrangement, of which examples are already known, in which a secondary coil or coils for detection purposes is or are interposed between the primary coil or coils and the material under examination.